No, it's on stable rust, maybe you got confused by the bug report talking about the nixos-unstable channel, that has nothing to do with the rust nightly builds, it's the version of nixos that is currently under development.
This is true for aarch64, but if you're not doing an embedded-linux style project, it's very likely you'll need nightly for inline assembly, though a significant chunk of that is being stabilized as we speak (though sadly not yet enough for my work...)
Good point. Though you can always link to .S for that last bit, right? I don't mean to tell you your business, I know you folks are doing much more intense stuff than I am. It doesn't surprise me that you'd know exactly what's keeping you from using stable.
See: https://doc.rust-lang.org/book/appendix-07-nightly-rust.html